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by: Erica Larson, News Editor The Horse.com
May 19 2011, Article # 18264The neurologic equine herpesvirus (EHV-1) outbreak in the western United States has captured the attention of horse owners across the country and raised a few questions about just what this virus is and what it might do in this current outbreak. TheHorse.com enlisted the help of Paul Lunn, BVMS, MS, PhD, head of the Department of Clinical Sciences at Colorado State University’s College of Veterinary Medicine, to explain the top five things horse owners should know about EHV-1.

1. Understand the Modes of Transmission

Understanding how EHV-1 is transmitted between horses is key to understanding how to slow or stop disease outbreaks.

“The most significant way the EHV-1 virus spreads is horse-to-horse contact, so right now the most important thing we can do is to stop this virus moving around,” Lunn said. “If we can stop this virus from moving from horse to horse, we can bring this outbreak to an end. Keeping horses at home and keeping them from meeting other horses they’re not already in contact with is the most critical strategy.”

He also noted that EHV-1 can survive on “surfaces, equipment, feeding tools, grooming supplies, trailers, or gates. For these reasons, we need to be aware that we can become the way the virus moves from horse to horse.”

Careful consideration when handling barn supplies, tack, and other stable supplies around exposed and nonexposed horses can reduce the risk of transmission.

Lunn said that EHV-1 can be transferred via aerosol transmission over short distances, but he added that unlike equine influenza–which can travel for several hundred yards through the air–a turnout arrangement that separates exposed and unexposed horses by a distance of a hundred yards or more would likely be sufficient to squelch transmission.

2. Know the Incubation Period

“The incubation period from when a horse first gets exposed to this virus (to when he starts showing signs of illness) can be quite lengthy,” Lunn said. “The horse can be febrile (temperature of 101.5 F or more) within a day of getting infected, but when the infection goes on to cause neurologic disease, that usually doesn’t develop for at least seven or eight days, and sometimes up to 14 days after it first gets infected.”

He explained that at-risk farms (farms housing horses that have traveled and might have been exposed at a show or an event, or farms that have had confirmed cases) should be quarantined for–at minimum–two weeks to account for the incubation period (cases that aren’t apparent yet). However, if there is evidence that a farm actually has EHV-1 infected horses, this needs to be longer. “Quarantine for those farms really needs to last at least three weeks and probably in many instances four weeks, for farms at which cases actually occur,” he noted. (This time begins after the last fever is detected on that farm.)

Lunn said that if an EHV-1 positive horse or one suspected of having the virus has been turned out with other horses, don’t mix those horses with nonexposed horses on the farm. Keep the horses in the same turnout groups to minimize exposure.

3. EHV Isn’t an Uncommon Virus

“Equine herpesvirus-1 is a very common virus to find in horses,” Lunn said. “The great majority of horses are exposed early in life and become lifelong latently infected (showing no signs of illness).”

The virus generally manifests as a respiratory infection and occasionally causes abortions in pregnant mares. The neurologic form of the virus, however, is an unusual outcome from the infection, he explained.

“What is most unusual right now is not that the virus is spreading, but that a very high number of horses are developing the neurologic disease.”

On the positive side, Lunn explained that when the current outbreak ends, the horses that were exposed and infected during the outbreak are unlikely to represent a threat in the future–the veterinary community has not seen these horses cause further outbreaks down the road.

4. Know the Early Signs

“The first thing we typically see with this disease is a fever,” Lunn said. “So if you see a fever with no obvious explanation, that should raise your anxiety level.

“And if you think there’s a chance your horse might have been directly or indirectly exposed and they have a fever, that would be an opportunity to be in touch with your veterinarian and discuss getting a nasal swab taken for PCR (polymerase chain reaction) diagnostic testing.”

Early detection of a fever could lead to an early diagnosis that will allow treatment to commence promptly. As with most equine ailments, early treatment generally leads to a more favorable prognosis.

5. EHV-1 Neurologic Disease is not a Death Sentence

“There are things you can do, get your vet involved, and be able to hopefully weather the storm,” Lunn said. “If you do have a horse that is EHV-1 positive, or worse yet, starts developing some signs of neurologic disease, there are definitely things you can do to give a good chance of improving the outcome of this disease. There are a variety of treatment options your veterinarian may use, but these include anti-inflammatory drugs and supportive care (i.e., the use of a sling, ’round the clock nursing, etc.) that can be given to help nurse that horse through the disease.

“One of the most important things a veterinarian will do is monitor whether the horse can urinate– often the paralysis can affect this and there is a real risk of bladder rupture–and keeping the bladder drained in this disease can be a life-saving procedure.” Lunn emphasized.

“We see many cases where the effects of the disease can be quite mild–just some stumbling or ataxia (incoordination),” he added. “Those horses can usually completely recover by a few months after the infection and go on to lead normal lives.”

Of course, not all cases are mild, and horses that are more severely affected generally don’t share the same fortune as mildly affected ones.

“Horses that are more severely infected and maybe end up on the ground or unable to rise have a much worse prognosis,” he noted.

Other Information

Owners might be wondering if a herpesvirus vaccination would be advisable during this outbreak. The answer, however, is not just a simple yes or no.

“It’s controversial,” Lunn explained. “We know that at the moment, we have no evidence that EHV-1 vaccines can stop the development of the neurologic disease. No vaccine has a claim right now for protection against neurological disease. We also know that some of the more potent EHV-1 vaccines can have a very significant effect on reducing the shedding of the virus from the nose in a well-vaccinated and recently boostered horse.”

A decrease in shedding EHV-1 will lead to a decrease in the number of horses exposed to the virus.

“So for these reasons, my inclination is to say if I had a horse that was previously vaccinated and it hadn’t had a booster within the last three months, I would give it a booster in order to decrease the movement and shedding of the virus on the farm,” he concluded, adding that this decision needs to be balanced against the need to “muster and handle” horses in order to give that vaccine. He added that not all experts agree that vaccination is entirely benign.

Another consideration is the well-known fact in the veterinary community that viruses hold the ability to mutate, but it’s too early in the course of the outbreak to determine if the current strain of EHV-1 is a mutated strain.

“At the moment, all that has been done is PCR molecular identification of the fact that the virus is present,” Lunn said. “I think a number of institutions–including CSU and the University of Kentucky–are trying to culture the virus right now, and in time we’ll be able to study its genetic code.

“Very likely, when this is all said and done, we will discover that this virus does possess the mutation that was characterized in the past decade, which we call the neuropathogenic mutation (i.e., strains of EHV-1 with a single mutation in the gene that encodes the enzyme called DNA polymerase). It’s very likely that this virus will carry this mutation. Now this virus might have other mutations and other changes in it that might eventually help explain why we’ve seen this disastrous outbreak, but at the moment that is speculative.”

Take-Home Message

Until the outbreak has settled, it is advisable to keep horses at their home base to reduce the risk of exposure to nonexposed horses. Additionally, understanding the virus and how it works is key to preventing further outbreaks.